FIG. 1 shows a typical prior art circuit in which two power MOSFETS 1 and 2 are connected in parallel to control a load 3, which may comprise a motor.
Each MOSFET 1 and 2 has an active clamp circuit comprising a zener diode and a diode, 5, 5′ and 6, 6′, respectively, connected between the gate and the common drain electrodes. The active clamp circuit serves to clamp the drain voltage at a specified level to prevent avalanche of the MOSFET.
In circuits of this type, a problem is encountered in that due to small variations in the voltage at which the clamp circuits operate, one of the parallel power MOSFET devices will essentially pass all of the current, or most of the current. With reference to FIG. 2, the clamp circuit for the device 1 and the clamp circuit for the device 2 have slightly different clamp voltages. The clamp voltage is typically determined by the zener diode 5, 6. The zener diode 5,6, will avalanche when the voltage across the zener diode exceeds the zener voltage, clamping the drain voltage. However, due to small variations, the clamp voltages of he two parallel devices may be somewhat different. As shown in FIG. 2, the graph of current versus voltage is very steep as the clamp clamps at the specified voltage and essentially maintains that clamp voltage. Accordingly, for the example shown in FIG. 2, the device 1 will pass most of the current because the clamp 6 for the device 2 does not activate, i.e., the zener diode 6 fails to avalanche because the drain voltage is clamped by the clamp circuit for the switch 1. As a result, substantially all of the current is passed by the device 1 and not the device 2 causing much more heating of the device 1. Thus, the parallel arrangement of the devices 1 and 2 fails to accomplish the goal of sharing current between the two parallel connected devices.